1. Field of the Invention
This invention relates in general to communication systems, and more particularly to an enhanced handoff signaling for high speed data and multimedia.
2. Description of Related Art
Cellular communication systems, operate on various air interface standards. For example, there is the first generation Analog Mobile Phone Systems (AMPS) whose air interface is described in detail in EIA/TIA (Electronics Industry Association/Telecommunications Industry Association) IS-553. Second generation standards include the U.S. Time Division Multiple Access (TDMA) standard, IS-54/IS-136, GSM (Global System for Mobile communications), and a Code Division Multiple Access (CDMA) standard, IS-95. In addition, there are standards for wireless local area networks that specify the air interface between stations and between stations and access points.
A major driver for the evolution to third generation cellular communication systems is the provision of high speed data and multimedia services. This implies that wireless networks must provide high speed packet-based and circuit-based transport and bandwidth-upon-demand, as well as support multimedia applications. Since the radio spectrum is limited, future wireless systems will have micro/picocellular architectures in order to provide the higher capacity needed to support high speed data services. Due to the small coverage area of micro/picocells and characteristics of the multipath and shadow fading radio environment, hand-off events in future microcellular systems will occur at a much higher rate as compared to today's macrocellular systems, and control of such systems will introduce a new set of challenges including handling the increased load on the wireless spectrum during handoff negotiations.
Wireless/mobile connections may be viewed as consisting of paths (or routes) through the backbone network; and radio links between the mobile, wireless terminals and base stations (or access points) which connect mobile users to the fixed backbone network through mobile switching centers (MSC), or functionally equivalent entities. In what follows, "wireless network" refers to the base stations and MSCs, or functionally equivalent entities.
When the quality of a radio link between a wireless terminal and its base station degrades, a new base station with acceptable quality must be found (hand-off), and network control functions need to be invoked in the wireless network, and possibly in the fixed network. Hand-off requires the establishment of a new route, which transports the user data destined to (or originated from) the wireless terminal to (or from) the new base station. Here, network call processing functions need to be invoked in order to set up such a route and ensure that the newly established route maintains acceptable quality-of-service (QOS) to both the wireless connection and to pre-existing calls sharing links of the new route. Furthermore, to execute hand-off, one must first ensure that the new wireless connection does not overload the new system and then create a radio link between the mobile terminal and the new base station. As one can see, a substantial number of call processing and control functions of the fixed and wireless network must be invoked to complete a hand-off event.
High speed data services require more complex service and air interface attributes than speech or lower speed data services. Service attributes are relevant to the calling party and called party's applications and are independent of air interface attributes. Examples of service attributes are an indication whether the bit rates are the same in both directions (symmetric) or not (asymmetric), the bit rate range, acceptable residual error rate, acceptable delay, etc. Air interface attributes are relevant only to the air interface. For example, in IS95, these attributes specify characteristics such as the multiplex option, rate set, etc. In essence air interface attributes provide the information required for building and interpreting traffic channel frames. With the advent of third generation systems, it is quite possible that there will be various types of base stations, i.e., base stations that support different sets of air interface attributes.
For convenience and conciseness, the collection of service attributes pertaining to a call are referred to as the Application Service Configuration (ASC). Herein below, ASC will be used interchangeably with service attributes. Note that the term "call" is used here in the broadest sense, and may include a multimedia conference or packet data session. The air interface attributes are referred to as High Speed Data Service Configuration (HSDSC). Herein below, HSDSC will be used interchangeably with air interface attributes. If a system does not have the required resources, the HSDSC may only meet the minimum requirements of the ASC, not the preferred ones. Examples of ASC and HSDSC will be discussed below. In what follows, in the most general case, "system" refers to a base station and the associated MSC.
Handoff between systems cannot be carried out unless a service negotiation take place so the target system and mobile terminal can agree on HSDSC that are compatible with their respective capabilities and also meet the service requirements, as indicated by the ASC. Service negotiation has to be efficient to minimize duration of service disruption caused by handoff, and minimize signaling load on spectrum resources.
One approach is to downgrade the radio channels to a default lower speed, carry out the handoff using the existing second generation handoff signaling procedure (for lower speed data), and perform service negotiation once the mobile is on the target system. After negotiation is completed, the air interface may be upgraded to the agreed upon higher bandwidth. The drawbacks of the method are a longer negotiation time, caused by the additional steps to downgrade and upgrade, and lower efficiency, caused by the possibility of the call being dropped due to unsuccessful negotiation after handoff is performed. Also, there is a heavier signaling load on the wireless link, where bandwidth is at a premium.
It can be seen that there is a need for an enhanced handoff signaling method that provides a short negotiation time by avoiding the need to downgrade and upgrade.
It can also be seen that there is a need for an enhanced handoff signaling method that provides greater efficiency by preventing calls from being dropped due to unsuccessful negotiation after the mobile station (MS) has moved to the target system.
It can also be seen that there is a need for an enhanced handoff signaling method that minimizes the signaling load on the wireless spectrum during handoff negotiations.
It can also be seen that there is a need for a target system to be capable of granting air interface attributes that more closely match the preferred service attributes, when the target system has the resources to do so, and when the air interface attributes currently in effect on the source system happens to not match the preferred service attributes.
It can also be seen that there is a need for an enhanced handoff signaling method that gives the option to switch to a new HSDSC, provided the new HSDSC is compatible with the mobile terminal's capabilities and meets the service requirements.